Many aerospace systems, such as engine exhaust ducts, nose cones, firewalls, and reentry shield surfaces, surfaces are exposed to high temperatures or large temperature gradients and must be insulated. Each application has unique problems which have rendered it difficult to provide an adequate thermal insulation that can be tailored for optimum performance.
Fiber insulation generally is weak and friable. It normally requires a surface cover to allow it to be handled without abrasion or erosion and to provide a toughened interconnecting surface for attaching the fiber insulation to other substructures. In U.S. Pat. No. 4,093,771, Goldstein discloses a reinforced fiber ceramic comprising a borosilicate glass coating on the surface of reusable silica insulation. In U.S. Pat. No. 4,381,333, Stewart discloses a two-layer glass coating for reinforcing silica insulation. The base layer has a high emittance and is preferably formed by combining a reactive borosilicate glass with an emittance agent, such as silicon tetraboride, silicon hexaboride, boron, or silicon carbide. The outer layer is formed from discrete, sintered glass particles to provide a high scattering coefficient. Preferably, fused silica or a reactive borosilicate glass having a higher silica content than the base layer is used for the outer layer. In either the Goldstein or Stewart ceramics, the coating is sprayed onto the underlying fiber insulation before firing to form a glass.
Another ceramic insulation can be formed witch an unsolidified silica glass felt sandwiched between silica glass fiber cloth. The three layers are stitched together with silica glass thread (or another suitable refractory thread) and are bonded with adhesive to the surface to be protected. A layering effect may be achieved by superposing a stitched blanket of silica and aluminoborosilicate fibers (commercially available under the trademark NEXTEL from 3M Company) over a separate, stitched blanket of silica fibers. By staggering the blankets and using suitable emittance coatings on the outer surfaces of the blankets, control of the insulative characteristics can be achieved, thereby countering the temperature distribution on and gradient through the insulation.
Fiberformed ceramic insulation with surprising physical properties is described in U.S. Ser. No. 698,496, and is made by forming a slurry of ceramic fibers, molding the slurry to form a soft felt mat, drying the mat, and incrementally introducing a sol-gel glass binder into the mat to form a rigid mat. The incremental addition of the sol-gel binder is accomplished through a unique multiple impregnation technique in which a small amount of binder is initially impregnated into the mat, is gelled, and is cured to stabilize the mat dimensionally, allowing handling and further processing of the mat. The mat is strengthened thereafter to its final strength by successive additions of glass binder. This technique cures the mat to a rigid, predetermined shape without appreciable shrinking of the resultant structure, and is contrasted with prior processes in which the entire binder is introduced either in one impregnation of the mat or by incorporating the binder in the fiber-containing slurry prior to the molding or felting operation. U.S. Pat. No. 3,702,279 to Ardary et al. and U.S. Pat. No. 3,935,060 to Blome et al. exemplify these prior processes.
A vacuum-felting process tends to align the ceramic fibers parallel to the forming surface, producing an anisotropic material having reduced flatwise tensile strength. This anisotropic material can be mechanically strengthened by stitching the mat with glass or other high-temperature refractory thread in a direction that is normal or at angles to the mat fibers. If the mat has layers, the stitching provides additional connection between the layers. Stitching can also be used to anchor the glass fabric of the coating to the mat.
The fiberformed insulation usually includes a network of ceramic fibers that are disposed in a plurality of layers, with fibers within each layer intersecting other fibers within the same layer. Some fibers within each layer intersect fibers in adjacent layers. To strengthen the layered network, sol-gel glass bonds are formed where the ceramic fibers intersect.
Fiberformed ceramics of this nature sometimes lack necessary toughness or flexural strength, especially at the point of attachment to the structural material. They can be improved, however, with reinforcing skins.
Ceramic parts that exhibit adequate strength and toughness for structural applications generally have been formed by hot press techniques that convert glasses to dense crystals, as described in U.S. Pat. No. 4,485,179. There, tows of fiber "tape" impregnated with glass powder were laminated and then consolidated at 1450.degree. C. and 1000 psi for 15 minutes in a vacuum. The hot press equipment is large in comparison to the part, because of the need to supply high temperatures, special atmospheres, and high pressures can be achieved. Difficulties arise in molding parts that include complex curvatures, and the conventional hot press equipment cannot accommodate large parts.